Electricity meters that measure energy consumption or power consumption typically connect between a utility power line and a load. For example, an electricity meter for a residential customer is often connected at the point at which the electrical system of the residence connects to the utility line. The meter may thereby perform measurements regarding the energy consumed by the load.
Utility meters often include one or more electrical contacts across which the load voltage and a significant amount of current may be found. For example, certain meters have switches that allow for controllable connection and disconnection of electrical service to the customer, i.e., the load. Such switches necessarily have substantial contacts to carry the entire customer load current, often in excess of 100 amps, during normal operation. There exists a potential for degradation of the switch contacts, particularly if the switches are used with some frequency. Degradation of the switch contacts increases the resistance over the switch contacts. Such resistance creates additional power loss within the meter, which is undesirable.
To address this issue, U.S. Pat. No. 7,683,642 shows a method and arrangement for monitoring meter contact integrity that determines a voltage drop across, or a resistance of, a contact associated with a utility meter. The method uses in part existing meter circuitry for determining voltages, currents and the like. FIG. 2 of U.S. Pat. No. 7,683,642 and the accompanying drawings shows how the resistance of a service switch contact may be determined using voltage measurements on either side of the switch, and the current measured on the line running through the switch.
Although U.S. Pat. No. 7,683,642 describes a suitable way to determine resistance of a service switch (and/or other electrical contacts) within an electricity meter, there is a need for higher accuracy. In particular, because of voltage drop across the contacts can be very small, and large common mode voltage, the tolerance of the measurement circuitry in some applications can cause significant errors in the contact resistance measurement.
In particular, a meter measurement circuit may determine voltage with a relatively tight tolerance of ±0.1% error. In such a circuit, the voltage measurement error on a 240 volt electrical system can be as much as 240 mV. Moreover, because contact resistance measurements involve measuring voltage on both sides a switch contact at approximately line voltage, the maximum error for a given measurement can be twice as much, or 480 mV. This error can be significant when measuring the relatively small voltage drops across a service switch or other low resistance element.
One solution to the issue is to employ high precision voltage measurement components such that the circuit has tighter error tolerance, for example, an order of magnitude better than 0.1%. Unfortunately, the cost of such components can add significantly to meter cost.
Accordingly, there is a need for method and arrangement for measuring contact resistance within a utility meter that has reduced error without necessarily implementing extremely high precision components.